The first total synthesis of leopolic acid A, a fungal metabolite with a rare 2,3-pyrrolidinedione nucleus linked to an ureido dipeptide, was designed and carried out. Crucial steps for the strategy include a Dieckmann cyclization to obtain the 2,3-pyrrolidinedione ring and a Wittig olefination to install the polymethylene chain. An oxazolidinone-containing leopolic acid A analogue was also synthesized. The antibacterial activity showed by both compounds suggests that they could be considered as promising candidates for future developments.

Mentions:
A straightforward route to the intriguing 2,3-pyrrolidinedione system appeared to be the Michael addition of a suitable amine to ethyl acrylate, followed by a Dieckmann cyclization with diethyl oxalate [10–11]. We chose the p-methoxybenzyl (PMB) protecting group for the amine, because of its facile cleavage with cerium ammonium nitrate (CAN) or 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). Thus, 2,3-pyrrolidinedione 3 was obtained by the reaction of ethyl acrylate with p-methoxybenzylamine, followed by treatment with diethyl oxalate (Scheme 1) [12]. On the basis of NMR data, the compound exists as an enol tautomer (see Supporting Information File 1). Indeed, perusal of the literature indicated that apparently all 4-monosubstituted 2,3-pyrrolidinediones are highly enolized [13–15]. Protection of the enolic OH with a benzyl group, using BnBr and K2CO3, gave compound 4. The reduction of 4 with DIBAL-H gave the corresponding primary alcohol, which was converted into bromide 5 by Appel reaction with PPh3 and CBr4. The phosphonium salt obtained from this bromide was subjected to a Wittig reaction with nonanal, to afford compound 6 [12].

Mentions:
A straightforward route to the intriguing 2,3-pyrrolidinedione system appeared to be the Michael addition of a suitable amine to ethyl acrylate, followed by a Dieckmann cyclization with diethyl oxalate [10–11]. We chose the p-methoxybenzyl (PMB) protecting group for the amine, because of its facile cleavage with cerium ammonium nitrate (CAN) or 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ). Thus, 2,3-pyrrolidinedione 3 was obtained by the reaction of ethyl acrylate with p-methoxybenzylamine, followed by treatment with diethyl oxalate (Scheme 1) [12]. On the basis of NMR data, the compound exists as an enol tautomer (see Supporting Information File 1). Indeed, perusal of the literature indicated that apparently all 4-monosubstituted 2,3-pyrrolidinediones are highly enolized [13–15]. Protection of the enolic OH with a benzyl group, using BnBr and K2CO3, gave compound 4. The reduction of 4 with DIBAL-H gave the corresponding primary alcohol, which was converted into bromide 5 by Appel reaction with PPh3 and CBr4. The phosphonium salt obtained from this bromide was subjected to a Wittig reaction with nonanal, to afford compound 6 [12].

The first total synthesis of leopolic acid A, a fungal metabolite with a rare 2,3-pyrrolidinedione nucleus linked to an ureido dipeptide, was designed and carried out. Crucial steps for the strategy include a Dieckmann cyclization to obtain the 2,3-pyrrolidinedione ring and a Wittig olefination to install the polymethylene chain. An oxazolidinone-containing leopolic acid A analogue was also synthesized. The antibacterial activity showed by both compounds suggests that they could be considered as promising candidates for future developments.